Patent GB-579896-A (GB)
Title: Improvements relating to railway traffic controlling apparatus
IPC: B61L3/22
Filed: 1944-02-18
Abstract: 579,896. Railway cab-signalling apparatus. WESTINGHOUSE BRAKE & SIGNAL CO., Ltd. (Allison, L. R., and Volz, C.). Feb. 18, 1944, No. 3051. [A Specification was laid open to inspection under Sect. 91 of the Acts, Sept. 17, 1944.] [Class 105] In a cab-signalling apparatus in which alternating current in the rails is periodically interrupted to constitute a code and operates an inductive receiver apparatus on the train having an electron valve amplifier to supply operating current to a codefollowing relay whereby, through de-coding apparatus a cab signal is given corresponding to the particular code the control grid of the valve is given a negative bias proportional to the voltage of the direct current supply source for the valve (locomotive generator) and a further automatic bias proportional to increase of rail current strength above the minimum operative value. This compensates for variations in the generator voltage and for the changes in rail current as the train progresses along a section. Alternating current flowing in opposite directions in the rails 1a, 1b acts inductively on the train-carried windings 2, 3, connected so that their voltages are additive, to energize a filter unit F1 comprising a transformer T1 with condensers 6, 7 for the primary and secondary windings ; a tapping 19 from the secondary winding is used for the automatic bias control of the control grid of the electron valve EV. The anode current of the valve passes to a transformer MT having a bypass condenser C2 for the alternating current component while the mean-value current, arising from a code pulse passes through the primary winding 23 to energize the secondary winding 28 to give alternating current of a frequency corresponding to the code and so to operate the polarized code-following relay CF at a like frequency. The bias voltage is such that with a fixed minimum operative rail current the mean-value anode-current pulses arising from the " on " periods of the code are sufficient to energize the secondary winding 28 of the transformer MT with alternating current to effect the operation of the polarized codefollowing relay. The electron valve EV is supplied by a train-carried generator G and is of the screen-grid indirectly heated type. The cathode 12 is connected to a tap point 16 between two resistances R2, R1 placed across the generator G. The 'control grid 13 is connected through the filter unit F1 and tapping 19 to a biasing unit BU comprising a condenser 21 and resistance 20 and thence through resistance R2 to the cathode 12. Thus it will be seen that any fall of generator voltage, so reducing the screen grid voltage, is accompanied by a reduction in the negative control-grid bias so that a compensation effect in the anode current is obtained. Further an increase in the alternating-current rail current sufficient to make the control grid positive at peak value would cause a control-grid current flow and so give an automatic increase in the negative bias by reason of the condenser 21 becoming negatively charged until a value was reached to stop any further control-grid current flowing. The time constant of the resistance-condenser combination 20, 21 is greater than the 11 off " period of the code and so the condenser does not become discharged during such " off " periods. Thus with increased rail current the negative controlgrid bias is automatically increased. The code-following relay CF operates a decoding unit DU comprising a pulsed transformer DT whereby relays A, R, L are respectively energized, the relays A, R through circuits DC1, DC2 tuned to the respective interruption frequencies, so that an appropriate aspect-light signal CS is illuminated in the cab. A reservoir condenser C1 is provided in parallel to the control-grid circuit to prevent a sudden change in the bias voltage (with consequent pulse in the primary winding 23 of transformer MT) if the control-grid circuit were to break (the condenser C1 would then only slowly discharge); thus an intermittent break would not give a false signal.